The present invention relates to magneto-optic storage media, and in particular, it relates to a direct-overwrite magneto-optic medium.
In magneto-optic recording, data is represented by a magnetized domain. A magnetized domain is a localized area which has a preferential magnetized orientation. Domain will herein refer to the smallest stable magnetizable region, although in common usage, a domain is a uniformly magnetized region of any size. The preferentially magnetized domain is often referred to as a "bit." The preferential magnetized orientation is caused by an energy source of sufficient intensity heating the medium above its Curie point temperature while the medium is simultaneously being biased by a magnetic field.
Typically, the energy source is a laser. The laser is used to heat the magnetic medium in the localized area. When the localized area exceeds the Curie temperature, then the magnetization direction is set by the magnetic field.
When the laser beam is removed, the bit cools in the presence of the biasing magnetic field and has its magnetization switched to that direction. The momentary temperature rise in the bit reduces the bit's coercive force so that the magnetic biasing field forces the magnetization of the domain to conform to the direction of the biasing magnetic field as the bit cools below its Curie point.
To write again on the recording media, conventional practice has been to erase what has been recorded. To accomplish the erasure, any given bit is exposed to a laser beam of sufficient intensity while also exposing that bit to a magnetic field in an opposition direction and permitting that bit to cool. This erasure step sometimes is referred to as an initialization step. The medium is then ready for writing on.
The conventional write-over procedure is constrained by two steps. The first being the erase or initialization step and the second being the recording or write-over step. There has been some work in attempting to provide a one-step overwrite system.
U.S. Pat. No. 4,794,560 (Bell et al.) describes an erasable magneto-optic medium having a first magnetic recording layer and a second magnetic biasing layer for providing a biasing field. Disposed between the first and second magnetic layers is a thermal isolation layer which provides a thermal barrier for controlling the temperature of the second magnetic biasing layer. When the medium is heated for a short time, the fringe field from the first layer causes one direction of magnetic orientation to occur in a domain in the first layer. Upon cooling of the domain, the domain magnetization becomes stable with an orientation conforming to the orientation of the fringe field. The thermal isolation layer prevents significant heating of the second magnetic layer. Further heating of the domain to erase, heats the second layer substantially to cause it to project the field opposite from the fringe field direction such that as cooling occurs, the orientation of the domain magnetization stabilizes and conforms to the projected field orientation.
U.S. Pat. No. 4,771,347 (Horimai et al.) describes a magneto-optic recording system which includes a magneto-optic recording layer and a biased magnetic layer. The magneto-optic recording layer has a Curie temperature higher than room temperature and the biased magnetic layer has a compensation temperature higher than the Curie temperature. A non-magnetic layer is disposed between the recording layer and the biased magnetic layer. A laser having first and second power levels is provided with the first power level being selected enough to heat the recording layer higher then the Curie temperature and to heat the biased magnetic layer higher than its compensation temperature, and the second power level being selected enough to heat the recording layer higher than the Curie temperature while keeping the biased magnetic layer below the compensation temperature of the biased layer.
West German published patent application DE 361,618 A1, and J. Saito, M. Sato, H. Matsumoto, H. Akasak, Tech. Digest and Int. Symp. on Optical Memory, WA-3 (Sept. 1987), describe a direct overwrite magneto-optic memory having a memory layer and a reference layer with the layers quantum exchange coupled. An initializing magnet is used to initialize the reference layer. At room temperature, the coercivity of the reference layer is larger than that of the memory layer and the Curie temperature of the reference layer is higher than that of the memory layer.